Spinal rods for immobilizing vertebral bones of the spinal column are typically anchored to the vertebrae via bone screws that extend through the pedicle into the vertebral bodies or by hooks that engage about the vertebrae. The rods are connected to the anchor members by generally yoke-shaped couplers that can be either integral with the anchor member head or separate components from the anchor member for use in polyaxial pedicle screw systems for spinal rod fixation. These prior systems employ some sort of compression member that is brought down into engagement either directly or indirectly with the spinal rod for securing it relative to the anchor member, and in polyaxial systems for securing the anchor member relative to the coupler.
For this purpose, the compression member and coupler typically are engaged via threading therebetween such that the compression member is threaded down into its locked position in or about the yoke-shaped coupler. Alternatively, wedge cam surfaces between radial flanges on the compression member and radial recesses in the coupler walls have also been employed to advance the compression member for pushing the spinal rod down into fixed position relative to the screw anchor member, see U.S. Pat. No. 6,565,565, to Yuan, et al. The problem with the threaded or cam wedge systems of spinal rod locking is that to allow the compression member to advance relative to the coupler, the size or profile of the coupler as well as the compression member necessarily is increased. In other words, to have threads or cam surfaces formed on the coupler requires that the walls be provided with a sufficient axial extent for the advancement of the threaded or cammed compression member therealong. Additionally, many systems utilize a full spherical head on a bone screw located adjacent the rod, thereby requiring a greater coupler to capture the rod and the bone screw head.
In polyaxial spinal fixation systems, the use of inserts between the head of the anchor member and the spinal rod has been proposed, see U.S. Pat. No. 5,733,286 to Errico, et al. The large hemispherical insert of Errico, et al. is engaged on a concave recess formed in a screw head received in the coupler allowing the coupler to adjust relative to the polyaxial pedicle screw for receipt of the spinal rod in its desired position. However, the entire Errico, et al. system has a undesirably large profile as it employs a threaded set screw for clamping on the spinal rod, and the hemispherical insert extends well beyond the top of the screw head into the coupler channel through which the spinal rod is received.